Introducing front projection

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Introducing front projection

Postby albertMunich » Wed Aug 09, 2017 7:54 pm

Do you remember Kubrick's "2001"?
The entire "Dawn Of Man"-sequence at the beginning that looks like it was shot outdoors was really staged on a giant sound stage in London. The African backgrounds were projected onto a huge 90 by 40 ft. front projection screen. This was one big innovation of the film, among many others.

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The front projection or "Scotchlite" process uses the retro-reflective capacities of 3M Scotchlite material . It concentrates light and reflects it back to the source in a very narrow angle. Traffic signs light up brightly when hit by car lights using the same materials. The stuff is also used for reflective strips on clothes. The most interesting material is Scotchlite 7610. It reflects light along the optical axis with 1610 times the brightness of a flat white wall.


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In closeup the material consists of microscopic glass beads with a reflective coating. Light enters the beads and is reflected in a 180 degree turn back to the source almost without loss. I did a lot of front projection experiments in the 1970s and have worked on several projects in the movies that used the process. BAVARIA studios in Munich at the time had an enormous 30 ft. front projection screen in one of their soundstages. On the opposite corner of the stage, I took out my cigarette lighter and looked at the screen through the flame. AND THE ENTIRE SCREEN LIT UP, just from that little lighter flame!
The material can still be bought, in the form of adhesive tape or as large rolls. It has a grey matte surface.
For the film process, a movie or still projector was set up next to the camera at 90 degrees to it. In front of the camera was a half silvered mirror at a 45 degree angle, reflecting side turned towards the screen. The axes of camera and projector lenses were made to coincide exactly. This way the shadows of the actors fell behind them and were masked by their own bodies. This alignment had to be done very precisely or a fringe or black line would appear around the foreground actors.

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My idea is that this process could be very useful in the context of NBTV. Nipkow disks could be made to reflect the images instead of passing them thru tiny holes. Beaded disks, as I have seen here on the forum could profit from this technique. This really squeezes every lumen out of a light source.If you had small transparent glass beads or pin heads instead of the holes in the disk, the light would travel back to the source, the LED array would have to come up to the front of the disk, nothing would have to pass THRU the disk anymore. The half silvered mirror should have a 70 /30 ratio -70 % transmission, 30 % reflectance. The scotchlite is so efficient it works even with a simple glass plate.
Other scanner shapes like a drum with strips of scotchlite could be realized.
Attached is my simple drawing with the N. disk , the LED Array pulled up front and the viewing lens. Would love to hear what you think of the idea.
Attachments
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Re: Introducing front projection

Postby Panrock » Thu Aug 10, 2017 7:13 am

Thanks for the very helpful dIagrams.

I didn't know glass beads had this property. For example (haven't checked) I thought the reflectors left behind on the Moon used a triangular arrangement. I guess the reflective coating behind the glass beads is crucial.

For an effectively straight-on light source like we have here, would the beads really be more efficient than simple mirror-finish pieces of foil, say?

Why the 70/30 ratio?

Otherwise, I can't see the advantage of this over a standard Nipkow Disc with holes and a flat light source directly behind. After all, the holes would have 100% transmission!

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Re: Introducing front projection

Postby albertMunich » Thu Aug 10, 2017 6:09 pm

The 70/30 mirror worked best for film work. 50/50 would lose you too much light from the foreground action, and a plain glass plate would cause double reflections. So the 70/30 struck the correct balance between foreground and intensity of the background.
My reasoning was that the light coming through the the hole in a Nipkow disc is immediately scattered in all directions but in the front projection system, almost all the light is bounced back straight to the source. total reflection. Besides that it opens up the possibility for creating a drum shaped scanner without the need to drill holes. As I have mentioned, these are thought experiments, I'm on holiday in France at the moment and have no access to my workshop and tools...

Scotchlite is very interesting. I used it for 3d as well, its so directional that two images in the correct distance get bounced back to the respective eye without any crosstalk at all. Made for the brightest 3d image of all times, but only for one viewer.

I had the chance to meet and talk with Jan Jacobsen who created the front projection system for SUPERMAN (flying scenes) and DAS BOOT. You can see the small projector under the ARRI Camera in the picture. That was almost 40 years ago...
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Using a 24 volt/250 W slide projector lamp and a handmade tiny projector sitting on the same tripod as the camera, he could light up a 20 to 30 foot screen with ease. It was that efficient. Try to get a strip of Scotchlite reflective material and shine a flashlight onto it from a distance. The closer you move the flashlight to the optical axis of the eye the brighter the reflection will be.
Jacobsen was a pioneer who created the first European 70 mm camera and later made the first IMAX camera that could run 65 mm film sideways with a 15 perforation wide pulldown without shredding the film. He was simply a genius.

http://www.in70mm.com/newsletter/1999/57/jacobsen/
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Re: Introducing front projection

Postby Panrock » Thu Aug 10, 2017 7:20 pm

Hi Albert,

albertMunich wrote:My reasoning was that the light coming through the the hole in a Nipkow disc is immediately scattered in all directions but in the front projection system, almost all the light is bounced back straight to the source. total reflection.

The light scattering starts at the light source. The Nipkow disc merely limits the light allowed through and also cuts down the angle of view since its holes are, in effect, 'little tunnels'. Equally, reflective hole equivalents would only catch a similarly tiny proportion of the total light and if the reflectors were directional, cut down the angle of view too... concentrating the light within that angle like a disc with tiny lenses. At least that's how it seems to me.

albertMunich wrote:Besides that it opens up the possibility for creating a drum shaped scanner without the need to drill holes. As I have mentioned, these are thought experiments, I'm on holiday in France at the moment and have no access to my workshop and tools...

I'm all for thought experiments! You can have drum scanners with lenses too.

albertMunich wrote:Scotchlite is very interesting. I used it for 3d as well, its so directional that two images in the correct distance get bounced back to the respective eye without any crosstalk at all. Made for the brightest 3d image of all times, but only for one viewer.

Now, that is very interesting!

Fascinating anecdotes. Thank you.

Steve O
Last edited by Panrock on Thu Aug 10, 2017 9:51 pm, edited 1 time in total.
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Re: Introducing front projection

Postby Klaas Robers » Thu Aug 10, 2017 9:50 pm

Steve, I agree with you. Holes in a Nipkow disc are small tunnels, as long as the diameter is larger than the wavelength of light. And that is the case: the holes have a diameter of about half a millimeter and the wavelength of light is about half a micrometer. So the holes are 1000 wavelengths wide. The result is that you are seeing a small part of the light source. If the light source is a point source, e.g. a clear LED without a diffuser in front, you would only see one point of light, that point of the NBTV picture.

On the other hand, a reflective disc has the problem of the too low attenuation (blackness) of the remaining part of the disc. You can never make the disc itself blacker than black paint. And the disc is illuminated at the front. This spoils the contrast (ratio of white / black in a checkerboard pattern) enormeously. This is too a problem of the back side of the mirror screw.

And indeed retroreflectivity works only for small glass piramids, not for spheres. However spheres have a somewhat similar effect, which you can see in the rainbow. Raindrops are small spheres, not "drops".
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Re: Introducing front projection

Postby Panrock » Thu Aug 10, 2017 9:57 pm

Klaas Robers wrote:On the other hand, a reflective disc has the problem of the too low attenuation (blackness) of the remaining part of the disc. You can never make the disc itself blacker than black paint. And the disc is illuminated at the front. This spoils the contrast (ratio of white / black in a checkerboard pattern) enormeously.

Good point.

Klaas Robers wrote:This is too a problem of the back side of the mirror screw.

Indeed it is, and this was a problem I had to solve as best I could. I ended up using roofing mastic, since not only was this black, it had superior adhesion to paint.

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Re: Introducing front projection

Postby Robonz » Wed Aug 16, 2017 9:07 pm

All you need to do is mount some active leds on your glasses and you are in business. Its very interesting idea. I spent years working with retro tape, it has come a long way.These days they actually have microscopic "corner cubes" molded into the tape. An incredible process. Glass bead retro is quite lossy but wider angle, so a good choice for your idea.

The problems I see with the design you posted is your head would need to be in the right spot to see the image.

There still might be an invention with retro and mechanical TV to be had yet, keep thinking.

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